In solid state memory, such as flash memory, data is typically stored in an array of cells using a charge trap or an isolated region of the cells. The cells may, for example, store one bit of information per cell in single-level cell (SLC) devices, or more than one bit per cell in multi-level cell (MLC) devices. A measured threshold voltage of the cell generally corresponds with a value of the data stored on the cell.
Data loss in solid state memory may occur for a variety of reasons including, for example, wear of cell materials and charge leakage. When such data loss is due to cell degradation, a shift in the threshold voltage may occur, resulting in data read errors of the degraded cell. Simply rereading or retransmitting the signal of the cell generally results in the same errors for degraded cells and fails to recover the data. Likewise, erasing and reprogramming the degraded cells generally results in similar errors.
As data density increases, a solid state memory device may rely on error correction code (ECC) for data integrity protection. In the case of an error correction code (ECC) decoding failure, a simple reread or retry of a degraded cell generally gives rise to little improvement over single read operations. Multiple reread and subsequent ECC decoding may be ineffective, for example, because the decoding failure may be a result of write-in error.